In general, power switches may be used to provide a switched power supply to an inductive load. Inductive loads typically resist a change in current, such that when the power switch is deactivated, the inductive load may operate as a current source to drive current to the switch. If the power switch is a metal oxide semiconductor field effect transistor (MOSFET) device that is driving an inductive load, the MOSFET device may be controlled by a driver circuit coupled to a gate terminal of the MOSFET device. In a particular example, the driver circuit applies a gate voltage of approximately zero volts to a gate contact that is coupled to the gate terminal. However, the inductive load may continue to drive current onto a drain terminal of the MOSFET device.
Generally, during a MOSFET on-state to off-state transition, a displacement current is discharged from the drain terminal of the MOSFET device via a drain-to-gate capacitance and through the driver circuitry. However, a gate resistance of the gate terminal increases with a distance from the driver circuitry. Thus, at a remote end of gate terminal, a gate resistance is greater than at an end closest to the driver circuitry. Assuming a uniform drain-to-gate capacitance, this non-uniform gate resistance will lead to a non-uniform localized gate voltage. Gate terminal regions with higher resistance will sustain a higher voltage potential, forming increased localized charge in the active channel of a MOSFET device. Thus, the non-uniform gate resistance can result in a non-uniform channel current during a switching operation from an on-state to an off-state. In a particular embodiment, the non-uniform channel current can include localized current crowding. In this example, the current from the inductive load may continue to flow through the active portion of the MOSFET device during the on to off-state transition, with localized channel regions handling an increased current density.
In a particular embodiment, a gate contact may be provided at a first end of the gate terminal and the peak channel current density of the MOSFET device may be shifted toward a second end of the gate terminal. In another particular embodiment, two gate contacts may be provided at opposing ends of the gate terminal, and the peak channel current density of the MOSFET device is shifted away from the gate contacts toward a center of the gate terminal. In general, the current at the drain of the MOSFET device is unevenly distributed across a width of the drain, resulting in a peak channel current density at a small portion of the MOSFET device, which may cause overheating and snap-back effects and which may damage the MOSFET device.